In a radio communication system such as a 3GPP (Third Generation Partnership Project) LTE (Long Term Evolution) Advanced system, a reduced throughput caused by interference at a user apparatus (UE: User Equipment) is a problem. The interference occurs, for example, in the following cases illustrated in FIG. 1:                a case where a user apparatus #1 residing in a connected cell (a serving cell of a connected base station) receives an interference signal by a radio beam from the connected base station to a user apparatus #2 on the same resource in MU-MIMO (multi-user multiple-input multiple-output), and        a case where the user apparatus #1 receives an interference signal from another cell as an interference cell (an interference cell of an interference base station).        
Therefore, in the LTE-Advanced, regarding downlink communication, technologies for reducing the interference have been studied in which the user apparatus suppresses the interference signal described above or removes (cancels) the received interference signal. As interference reduction technologies, there are, for example, an interference rejection combining (IRC) reception and a successive interference cancellation (SIC).
<Interference Rejection Combining (IRC) Reception>
The IRC reception is a technology in which a desired signal is obtained by separating the desired signal from a reception signal which includes an interference signal and the desired signal. The IRC is a technology in which, regarding the downlink communication, a corresponding weight (reception weight) is applied to each signal obtained at each of reception antennas in order for the user apparatus to suppress interference directed to the desired radio beam from the connected base station created by interference radio beams from the interference base stations and by signals transmitted to other users by the connected base station.
The reception weight is calculated from a formula derived from a minimum mean squared error (MMSE) algorithm shown in FIG. 2. It should be noted that there are two methods (Type1, Type2) for calculating the weight: a method in the case where it is possible to estimate a channel of the interference signal and a method in the case where it is not possible to estimate a channel of the interference signal. The formula shown in FIG. 2 is a formula in the case where it is possible to estimate a channel of the interference signal (Type1). It should be noted that the technology in which the reception weight is calculated by using the formula shown in FIG. 2 is a known technology.
In the formula shown in FIG. 2, the underlined portion is a covariance matrix which includes a channel matrix of the interference cell. Further, the channel matrix of the interference cell is obtained by performing channel estimation using a reference signal from the interference cell. Here, the channel estimation uses, for example, a two-dimensional MMSE channel estimation filter described in non-patent document 1. It should be noted that in LTE Advanced, the reference signals which can be used for the channel estimation include a Cell-specific Reference Signal (CRS), a CSI Reference Signal (CSI-RS), a Demodulation Reference Signal (DMRS), etc.
<Successive Interference Cancellation (SIC)>
SIC is a technology for separating the desired signal from a reception signal which includes an interference signal and the desired signal. The SIC is a technology in which the desired signal is extracted by: creating a replica signal of the interference signal from the received signal by using hard decision or soft decision; and successively subtracting (removing) the replica signal from the received signal. A functional configuration of a user apparatus which performs SIC is shown in FIG. 3. As shown in FIG. 3, the user apparatus performs a process of SIC by, for each of a plurality of interference signals, performing channel estimation of the interference signal, demodulating the interference signal based on the channel estimation, creating a replica, and subtracting the replicas successively from the received signal.
As described above, in order to perform IRC reception or SIC, it is necessary for the user apparatus to perform channel estimation of the interference signal.
Further, in 3GPP Rel. 12, Dynamic TDD (Time Division Duplex) in which uplink/downlink resource configurations of multiple cells are dynamically changed, and device-to-device (D2D) communications have been studied.
<Dynamic TDD>
In TDD of the conventional LTE, in order to suppress interference between the uplink and the downlink, as shown in FIG. 4(a), a cell #1 of a base station #1 and a cell #2 of another neighboring base station #2 have the same resource configuration (uplink/downlink/Special Subframe). In an example of FIG. 4(a), for each of the subframe numbers “0” through “9”, the resource of the cell #1 and the corresponding resource of the cell #2 are the same. Further, the resource configurations are changed at a long time period (e.g., from at a several-hour period to a one-day period) according to traffic situations of the cells (cell #1, cell #2).
On the other hand, in Dynamic TDD, as shown in FIG. 4(b), in the multiple cells (cell #1, cell #2), the uplink/downlink resource configurations are dynamically changed according to the traffic situations, etc. In an example of FIG. 4(b), different resources are assigned in different cells. For example, the resource of the cell #1 corresponding to the subframe number “3” is “uplink”, while the resource of the cell #2 corresponding to the subframe number “3” is “downlink”. Further, in this example, a situation is shown in which, for the subframe numbers “0” through “9”, more resources are dynamically assigned to downlink in the cell #2 because there is more downlink traffic in the cell #2. In this way, resources can be efficiently used in Dynamic TDD.
In Dynamic TDD, however, interference occurs between the uplink and downlink because different cells have different resource configurations.
<Device-to-Device (D2D) Communications>
As shown in FIG. 5, it has been studied to use the uplink in TDD for device-to-device communications between a user apparatus #2 and a user apparatus #3. It should be noted that although in an example of FIG. 5, a case is shown in which FDD (Frequency Division Duplex) is used for communications between the base station and the user apparatuses, TDD may also be used.
Therefore, the user apparatus #1 which is near the user apparatuses #2 and #3 performing the device-to-device communications receives an uplink interference signal used for the device-to-device communications.
In this way, there is a problem that if the uplink signal, used for Dynamic TDD or the device-to-device communications, is received while a user apparatus is receiving the desired downlink signal, the uplink signal causes interference and the downlink reception quality at the user apparatus is degraded.